Commercial vehicle transmissions, for example, automatic power-shift transmissions for city busses, are often equipped with a hydrodynamic torque converter as the start-up element for reasons related to comfort and wear, and are often equipped with an additional wear-free constant-braking device, such as a hydrodynamic retarder, because of the thermal load on the service brakes resulting from a plurality of start-up and braking procedures occurring in close succession or resulting from downhill travel with a high overall weight. This results in additional costs and complexity, however, due to components, weight, and construction space. Conventional hydrodynamic retarders also generate braking torque in the deactivated state, which is additional drag torque and increases the overall drag torque of the vehicle transmission and therefore unfavorably affects fuel consumption and the efficiency of the drive.
In hydrodynamic retarders, the mechanical energy of a drive shaft is converted into the kinetic energy of a hydraulic fluid, for example, oil. The physical operating principle corresponds to that of a hydrodynamic clutch, which comprises an impeller driven by an engine, as the drive, and a turbine as the output, wherein the turbine is fixed. A conventional hydrodynamic retarder therefore comprises a rotor blade wheel located in the power flow and a stator blade wheel, which is fixedly connected to a retarder housing. When the retarder is actuated, a quantity of fluid corresponding to the desired braking power is introduced into a retarder chamber. The flow stream is regulated by means of an electric proportional valve, for example, the proportional magnet of which is energized accordingly. In the retarder chamber, the rotating rotor propels the hydraulic fluid, which is supported against the stator blades, whereby kinetic energy of flow is converted into heat and a braking effect is thereby generated on the rotor and the driving shaft thereof, thereby braking the entire vehicle.
Start-up and retarder elements that combine the functions of a hydrodynamic start-up element, such as a fluid coupling or a torque converter, and a hydrodynamic retarder in one unit are already known.
Such a system, which is known from DE 100 45 337 A1, comprises a hydrodynamic clutch having an impeller and a turbine, wherein the impeller is connected to a drive motor, and a friction clutch is engaged in parallel for the purpose of lock-up. In addition, the turbine is connected to a transmission input of a downstream manual transmission by means of a freewheel and can be fixed on a housing by means of a turbine brake. At start-up, the drive power is transferred to the transmission input by means of the hydrodynamic circuit. In order to implement braking, the turbine is fixedly braked against the housing and the friction clutch is engaged. Filling the hydrodynamic clutch with hydraulic fluid permits the hydrodynamic clutch to function as a primary retarder.
EP 0 879 370 B1 discloses a transmission unit comprising a hydraulic transmission part, which has a primary blade wheel and a secondary blade wheel, which together form a working chamber, which can be filled with hydraulic fluid, and comprising a mechanical transmission part disposed downstream thereof. The mechanical transmission part can be, for example, a planetary transmission having one or more coupled planetary gear sets, with forward gear steps and reverse gear steps. The hydraulic transmission part can be operated in two operating states, namely in a first driving state as a hydrodynamic clutch and in a second braking state as a hydrodynamic retarder.
In the driving state, for example in a start-up procedure, power is transferred from a primary blade wheel to the mechanical transmission part via a secondary blade wheel. In this case, the primary blade wheel functions as an impeller and the secondary blade wheel functions as a turbine. In the braking state, one of the two blade wheels is fixed and the other of the two blade wheels is connected to the mechanical transmission part. In this case, the primary blade wheel functions as a stator blade wheel and the secondary blade wheel functions as a rotor blade wheel, which rotates in the reverse direction due to the opposite direction of flow. A plurality of power-shift elements embodied as clutches or brakes is provided for the two operating states, each of which can act on the blade wheels, optionally together with further shift elements of the mechanical transmission part, and can couple one blade wheel or both blade wheels to the transmission, or can bypass or fix one of the blade wheels or both of the blade wheels. A driving state is implemented by means of either a forward gear stage or a reverse gear stage with the start-up retarder disengaged or bypassed, and a braking state is implemented in each case by means of a reverse gear stage of the transmission with the primary blade wheel fixed.
Start-up retarders in which a hydrodynamic retarder is combined with a planetary gear set are also already known. As a particularly convenient replacement for a start-up clutch, the start-up retarders permit a hydraulic start-up procedure to be implemented with an additional start-up gear ratio and in a retarder mode. In order to ensure effective operation of such a design, however, both directions of rotation of the blade wheels must be taken into consideration, since, in a start-up procedure using such a retarder, the relative speed of rotation between the rotor and the stator can be negative, given that the rotor rotates in reverse, whereas this relative speed of rotation is positive in the actual retarder mode.
In such a start-up retarder, the rotational speed differential between the rotor and the stator tends to move toward zero in a start-up procedure. At an operating point at which the start-up retarder has reached the so-called parabolic peak thereof, and when the retarder chamber is filled with fluid and the fluid density and, therefore, the pressure cannot be increased any further, the transferrable torque begins to decrease quadratically with the rotational speed differential. Since the fluid density, together with the rotational speed differential, is a determining factor for the transferrable torque, a suitably disposed friction-shift element is usually required, which is activated in the engagement direction in order to compensate for the torque reduction and to conclude the start-up procedure.
Such a start-up retarder, comprising a hydrodynamic retarder and a planetary gear set, is known from DE 198 17 865 A1. The hydrodynamic retarder comprises a rotatable rotor blade wheel and a fixed stator blade wheel. The planetary gear set comprises a ring gear, a sun gear, and a planet carrier having planetary gears. The rotor of the retarder is connected or connectable to the sun gear and, in the reverse direction, is connectable or connected to the planet carrier. The planet carrier is connected or connectable to an engine-side drive shaft or to a transmission-side output shaft of the planetary gear set. The ring gear is therefore connected either to the output shaft or to the drive shaft. The retarder is designed as a double-flow retarder. This comprises two flow circuits having two blade assemblies, which are oppositely slanted relative to the circumferential direction, thereby enabling the rotor to generate sufficient braking power in both possible directions of rotation thereof.